You can get discounts online, but only though Korean websites.

You can also see some older women selling discount tickets inside Jamsil station, but they won't speak English, and are probably not willing to do business with a foreigner (they are discrete as they are scalping tickets) and interacting with a foreigner will only draw attention to themselves.

They used to have a promo where if you donated an old mobile phone to charity, you could buy a discount ticket, but that was a couple years back, not sure if its still offered.